1. Field of the Invention
The present invention relates to a photomultiplier of so-called side-on type into which light to be measured is incident through a side of a container.
2. Related Background Art
FIG. 1 is a side view, partly in vertical section, of a conventional side-on type photomultiplier which is generally used, and FIG. 2 is a cross-sectional view of the photomultiplier. In this photomultiplier, light to be measured enters through a side of a glass bulb 1 which is a transparent closed container. The incident light passing through the glass bulb 1 impinges on a photosurface of a reflection type photocathode 2, whereby photoelectrons are emitted from the photosurface. The photoelectrons are then delivered to an electron multiplying unit constituted of plural stages of dynodes 3a, 3b, 3c . . . . The electron multiplying unit successively multiplies the photoelectrons, and the multiplied electrons are collected as an output signal in an anode 4.
A grid electrode 6 is provided between a light entrance portion 5 of the glass bulb 1 and the photocathode 2 so as to guide the photoelectrons emitted from the photocathode 2 to dynode 3a of the first stage. The potential of the grid electrode 6 is set to be equal to that of the photocathode 2. There are various types of grid electrodes which may be employed as the grid electrode 6. For example, the grid electrode 6 may be a grid electrode (not shown) constituted in a manner that fine conductive wires are placed in a grid-shaped configuration, or a grid electrode constituted in a manner that one fine conductive wire 6c is helically wound around two supporting rods 6a and 6b as shown in FIG. 1.
There is also known a side-on type photomultiplier disclosed in JP-B-53-18864. As shown in FIG. 3, in this side-on type photomultiplier, a glass plate 7 on which a transparent conductive film is formed is employed instead of the grid electrode 6.
There is also known a side-on type photomultiplier disclosed in JP-A-4-292843. JP-A-4-292843 discloses a structure in which a conductive portion such as an aluminum-evaporated film is formed on an inside wall surface of a glass bulb except for a light entrance portion. Further, JP-A-4-292843 also discloses that the conductive portion is formed also on the light entrance portion when the conductive portion is transparent. The conductive portion reduces a resistance of the inside wall surface of the glass bulb, so that a time constant formed by stray capacitance and the surface resistance of the inside wall surface of the glass bulb is small. Since the time constant is small, the unstableness of the potential on the inside wall surface of the glass bulb is eliminated. As a result, an influence upon an electron track of photoelectrons is reduced, whereby a hysteresis characteristic is improved. The hysteresis is a phenomenon that an output signal rises not suddenly but gradually to reach stability when an optical pulse enters a photomultiplier.